Theoretical evaluation of the temperature dependence of magnetic anisotropy constants of Nd₂Fe₁₄B: Effects of exchange field and crystal field strength

To identify the possible mechanism of the coercivity (H_c) degradation of Nd-Fe-B sintered magnets, we study the roles of the exchange field acting on the 4f electrons in Nd ions and theoretically investigate how variation in the exchange field affects the values of the magnetic anisotropy constants K₁ and K₂. We find that, with decreasing exchange field strength, both values decrease as a result of the lower asphericity of the 4f electron cloud, indicating that the local anisotropy constants might become small around the grain boundaries, where the exchange fields are reduced owing to the smaller coordination number.